Field
The present disclosure relates to protection systems and methods of using the same in a nuclear reactor.
Description of Related Art
In a conventional liquid metal cooled reactor, a high primary and secondary coolant flow rate (>90%) is established before bringing the reactor core to a critical state. To increase electrical power to the grid, the outlet of the core temperature is increased, thereby “pushing” more heat out of the power system. This method brings about two problems: expensive pumping loads at low power and large thermal variations during changes in reactor power (induces thermal stresses across the fluid system). These two issues inhibit the system's ability to responsively load follow as designed. These problems lead to higher O&M costs to the plant owner.
In particular, in a conventional liquid metal cooled reactor, temperature is increased to increase power output to the grid. There may be about a 175° C. temperature change when the reactor goes from 0 to 100% power. If the plant is used for load following, this will induce a large number of thermal cycles. Thermal cycles in sodium, also known as thermal stripping, cause unnecessary wear on metal components. Furthermore, at low power operations, the primary and secondary pumps must still operate at 100% flow, which consumes a great deal of power. For example when PRISM is operating at 100% power and 100% flow, the primary and secondary pumps consume approximately 7% of the reactor's electrical output. This percentage increases as reactor power is decreased because sodium flow must remain constant.
Nuclear reactors also use a variety of damage prevention/mitigation devices to prevent core damage. These devices typically center on a control system that senses a “problem” and then activates the protection system. An important aspect of risk mitigation is the prevention of core damage in the seconds after loss of primary coolant flow. A gas expansion module (GEM) is conventionally used to protect the core of a liquid metal cooled reactor in the event of pump failure. The GEM has two operating states: one where it is voided of sodium (without flow) and one where it is full of sodium (at high flow). The GEM is an inverted tube filled with helium gas to a specific density. The GEM communicates with the core inlet plenum (the highest pressure point in the system). Without pump flow, the gas expands almost to the bottom of the GEM creating a voided assembly at the outside of the core. During plant startup, pumps are energized to provide flow to remove fission heat. The high pressure coolant in the inlet plenum compresses the gas in the GEM filling most of it with sodium, thereby reducing neutron leakage from the core by reflection of the neutrons. The GEM can have reactivity effects on the core if the helium gas is not compressed high enough in the GEM. The issue requires the primary coolant flow to be established at or above 90% rated flow. A filled GEM at this core flow will not induce power oscillations with normal flow oscillations. If the pumps turn off, for any reason, the compressed gas expands and reestablishes the void. This increases the neutron leakage which inserts negative reactivity. This negative reactivity brings the reactor to a subcritical state.